1. Field of the Invention
The present invention relates to charging circuits for batteries, and in particular to a charging circuit for fast charge batteries having a timer means therein.
2. Description of the Prior Art
The use of rechargable fast charge batteries for operating hand-held appliances, such as drills and other power tools, is well known. Nickel-cadmium batteries possess relatively rapid recharging characteristics, and are therefor well suited for such use. A problem with such nickel-cadmium batteries, however, is that such batteries can safely be charged at a normal charging rate only while the temperature of the battery is below a certain temperature. When that temperature is reached, the batteries should be charged at a lesser rate, typically approximately 1/10th of the normal charging rate, in order to "top off" the charge of the batteries allowing them to cool and reach the maximum charge. The lesser charging rate is known as the "trickle mode" charge and in this mode the batteries may remain connected to a power source for months without suffering any degredation in performance. Failure to switch to the trickle mode charge by maintaining the normal charge rate will, however, result in venting of the batteries, which may result in destruction of the batteries or may drastically shorten the useful life of the batteries.
In recognition of the necessity to thermally monitor the recharging nickel-cadmium battery packs, conventional battery operated tools have circuitry generally as shown in FIG. 1. The circuit includes a charger 1 which is supplied with 120 VAC, and furnishes an out-put of 7.5 VDC. One out-out terminal of the charger is connected to a battery pack 2 to be charged. The battery pack is in physical contact with a thermostat 3, permitting heat transfer from the battery pack to the thermostat. A power resistor 4 is in physical contact with the housing or shell of the thermostat, also permitting heat transfer there between. The resistor 4 is connected to the remaining terminal of the charger 1. When the battery pack 2 is at room temperature, the thermostat is normally closed. When the charger 1 is connected to a 120 VAC source, the charging current flows through the normally closed contact of the thermostat 3 and charges the battery pack 2. When the batteries reach a charge close to the full charge, the temperature of the battery pack 2 increases causing the thermostat 3 to open. In conventional circuits, the thermostat 3 is set to open at approximately 113.degree. F. (45.degree. C.). When the thermostat 3 opens at this temperature, current now flows through the power resistor 4. The resistor 4 is typically a 20 ohm resistor with 0.25 watt power dissipating capacity. When such a resistor is utilized, the current therethrough drops to approximately 0.135 amp. The power dissipated by the resistor 4 is thus (0.135).sup.2 .times.20=0.36 watt. Because 0.36 watt is larger than 0.25 watt, the temperature of the resistor 4 is elevated, and maintains the temperature of the thermostat above the threshhold temperature (that is, typically above 113.degree. F.). The thermostat 3 thus remains open even as the battery pack 2 cools, so that trickle mode charging is maintained indefinitely.
A problem associated with conventional charging circuitry of the type described above is that batteries such as nickel-cadmium batteries become elevated in temperature when discharged very rapidly, causing the thermostat 3 to open. Thus, when the batteries are shortly thereafter reconnected to a power source for recharging, the trickle mode charging will automatically be in effect and the batteries will reach full charge in approximately 10 hours, instead of the normal charging time of 1 hour. For this reason, manufacturers of hand tools utilizing batteries of this type, generally recommend that the tool be permitted to cool for approximately 1/2 hour before connecting it to a power source for recharging. Frequently, the user will in the meantime become occupied elsewhere, and forget to connect the tool to a recharging source after the passage of the designated time. Moreover, the user may have to estimate the passage of time, and cannot be positive that at the time of connection of the tool to the power source, the battery pack has cooled sufficiently to insure that trickle mode charging is not taking place.